We’ve all seen lemon batteries or potato clocks, but have you ever seen a water battery?
[Nathan Stubblefield] was an American inventor (born 1860) who never got quite as much recognition as some of the other great inventors of the time, [Tesla, Bell, Edison etc] — though he did demonstrate some very interesting wireless telephony technology. In addition to dabbling with invisible radio waves, [Stubblefield] filed a patent for something called an Earth Battery, which makes use of two coils of dissimilar materials (a voltaic couple) submerged in water (or moist earth). As you can imagine, it wasn’t overly effective, nor efficient by any means — but it worked.
[Lasersaber] has been playing around with the “Stubblefield Coil” recently, and designed a working flashlight using the theory. He designed a 3D printed coil holder which allows you to easily wrap copper and magnesium strips around it to create the coil. Three of these cells go together in series to produce your water battery (and handle of the flashlight).